The quality of a signal in a communication system is affected by the amount of distortions introduced by the system during transmission. In a transmitter, and in particular, a power amplifier (PA) is one of the main sources of nonlinear distortions in the wireless radio transmission link. A plot of the output power of a PA as a function of input power is called the amplifiers compression curve 102, shown generally in FIG. 1. As input power increases and the output power of the amplifier is driven closer to its maximum level, this relationship becomes non-linear. This non-linear region 104 is usually referred to as the compression region where equal increments of input power result in smaller increments of output power. The gain 106 of the amplifier drops off in this region. Unfortunately, in order to maintain high power efficiency, the PA must usually be operated in the compression region, which introduces considerable distortions to the signal.
The power spectral density of a transmitter's output is sometimes referred to as the spectral emission. Nonlinear amplification of a signal may cause the signal to spread in the frequency′ domain into adjacent frequency bands resulting in so-called extraneous emissions. Most communications standards set a limit on the spectral emissions of a transmitter. The shape of the spectral emission of the transmitter i.e. the emitted power density as a function of frequency is determined by the specific type of signal modulation, as well as a statistical distribution of a baseband digital bit sequence. Given this information the shape of the spectral emission can be determined for an ideal system. However system non-idealities skew this shape. The PA non-linearity is one of the major sources of this non-ideality, which often tends to widen the bandwidth of the transmitted signal and raise the power density level in the frequency regions neighboring the transmission channel (spectral regrowth). A spectral mask is defined by standards (such as those by the 3rd Generation Partnership Project (3GPP)) which sets a limit on the spreading of the power density spectrum as well as extraneous emissions such as harmonics of the carrier frequency or any unwanted spurious signals generated by the amplifier.
A PA's nonlinearity is sometimes evaluated using a metric called the adjacent channel power ratio (ACPR). The ACPR is defined as the ratio of total power within a certain bandwidth separate from the transmission channel, usually coinciding with the channel adjacent to the transmission channel to the total power within the transmission bandwidth.
In order to obtain an acceptable Bit Error Rate (BER) at the receiver end, linearization techniques may be used to compensate for these distortions while maintaining high power efficiency. Among the various methods for compensation, commonly used linearization techniques include digital predistortion (DPD) and post-compensation at the receiver side. Without linearization, power back-off at the transmitter is required to obtain an acceptable BER, assuming that the channel is well equalized at the receiver side and that the receiver is fully linear.
Pre-distortion is basically a method by which one first stimulates a non-linear PA with baseband samples and then observes the result of that stimulus at the PA output. Then, the amplitude-to-amplitude modulation (AM/AM) and amplitude-to-phase modulation (AM/PM) effects of the PA are estimated. These estimated distortions are then removed from the PA by pre-distorting the input stimulus with their inverse equivalents. In other words DPD consists of applying an inverse function having a gain with an inverse amplitude and phase behavior of the complex gain behavior of the PA to the signal before sending it to the power amplifier. The cascaded pre-distortion function and the PA have a combined gain that is linear particularly in the compression region. In order to compensate for the complex gain distortion of the PA close to saturation, the amplitude and phase are expanded relative to the original input signal which causes an increase in the signal peak-to-average power ratio (PAPR). Since the maximum power of the input signal is limited by the PA saturation, an increase in the PAPR forces a decrease in the mean input power (typically 2-4 dB). This power back off results in decreased drain efficiency of the PA transistor(s). On the other hand, DPD performance is considerably affected by impairments in up and down-conversion circuits (such as feedback path components for example mixers, filters, quadrature modulator and demodulator). These impairments affect estimation of the inverse function of the PA behavior.
Other techniques for DPD use training sequences to characterize the PA nonlinearity. In this approach, a training sequence is padded in each frame and is sent to the receiver. At first, the effect of the channel is equalized. The resulting signal is then considered as the replica of the signal at the output of the PA. Based on the training sequence, which is known at the receiver side, the PA nonlinearity is extracted. The model is then sent to the transmitter for compensation.